Spiral Cleavage Research Articles

Trochophora larvae: Cell‐lineages, ciliary bands, and body regions. 1. Annelida and Mollusca

The trochophora concept and the literature on cleavage patterns and differentiation of ectodermal structures in annelids ("polychaetes") and molluscs are reviewed. The early development shows some variation within both phyla, and the cephalopods have a highly modified development. Nevertheless, there are conspicuous similarities between the early development of the two phyla, related to the highly conserved spiral cleavage pattern. Apical and cerebral ganglia have almost identical origin in the two phyla, and the cell-lineage of the prototroch is identical, except for minor variations between species. The cell-lineage of the metatrochs is almost unknown, but the telotroch of annelids and the "telotroch" of the gastropod Patella originate from the 2d-cell, as does the gastrotroch in the few species which have been studied. The segmented annelid body, i.e. the region behind the peristome, develops through addition of new ectoderm from a ring of 2d-cells just in front of the telotroch. This whole region is thus derived from 2d-cells. Conversely, the mollusc body is covered by descendants of cells from both the C and D quadrants and a growth zone is not apparent. This supports the notion that the molluscs are not segmented like the annelids, and that the repeated structures seen in polyplacophorans and monoplacophorans do not represent a segmentation homologous to that of the annelids.

The MAPK cascade in equally cleaving spiralian embryos

Spiralian development is shared by several protostome phyla and characterized by regularities in early cleavage, fate map, and larva. Experimental evidence from multiple spiralian species implicates cells in the D quadrant lineage as the organizer of future axial development of the embryo. However, the mechanisms by which the D quadrant is specified differ between species with equal and unequal spiral cleavage. Equally cleaving mollusc embryos establish the D quadrant via cell–cell interactions between the micromeres and macromeres at the 24- to 36-cell stage. In unequally cleaving embryos, the D quadrant is established at the 4-cell stage via asymmetries in the first 2 cell divisions. We have begun to explore the molecular mechanisms of D quadrant patterning in spiralians. Previously, we showed that, in the unequally cleaving embryo of the mollusc Ilyanassa obsoleta, the MAPK pathway is activated and functionally required in 3D and also in the micromeres known to require a signal from 3D. Here, we examine the role of MAPK signaling in 4 spiralians with equal cleavage. In 3 equally cleaving molluscs, the chiton Chaetopleura, the limpet Tectura, and the snail Lymnaea, the MAPK pathway is activated in the 3D cell but not in the overlying micromeres. In the equally cleaving embryo of the polychaete annelid Hydroides, MAPK activation was not detected in the 3D macromere but was observed in one of its daughter cells, 4d. In addition, inhibiting Tectura MAPK activation disrupts differentiation of 3D and cells induced by it, supporting a functional role for MAPK in axis specification in equally cleaving spiralians. Thus, MAPK signaling may have a conserved role in the D quadrant organizer cell 3D in molluscs. However, there have been at least 2 evolutionary changes in the activation of the MAPK pathway during spiralian evolution. MAPK function in the Ilyanassa micromeres is a recent cooption and, since the divergence of annelids and molluscs, there has been a shift in onset of MAPK activation between 3D and 4d. We propose that this latter shift correlates with a change in the timing of specification of the secondary embryonic axis.

Conserved Mechanism of Dorsoventral Axis Determination in Equal-Cleaving Spiralians

Many members of the spiralian phyla (i.e., annelids, echiurans, vestimentiferans, molluscs, sipunculids, nemerteans, polyclad turbellarians, gnathostomulids, mesozoans) exhibit early, equal cleavage divisions. In the case of the equal-cleaving molluscs, animal–vegetal inductive interactions between the derivatives of the first quartet micromeres and the vegetal macromeres specify which macromere becomes the 3D cell during the interval between fifth and sixth cleavage. The 3D macromere serves as a dorsal organizer and gives rise to the 4d mesentoblast. Even though it has been argued that this situation represents the ancestral condition among the Spiralia, these inductive events have only been documented in equal-cleaving molluscs. Embryos of the nemertean Cerebratulus lacteus also undergo equal, spiral cleavage, and the fate map of these embryos is similar to that of other spiralians. The role of animal first quartet micromeres in the establishment of the dorsal (D) cell quadrant was examined in C. lacteus by removing specific combinations of micromeres at the eight-cell stage. To follow the development of various cell quadrants, one quadrant was labeled with DiI at the four-cell stage, and specific first quartet micromeres were removed from discrete positions relative to the location of the labeled quadrant. The results indicate that the first quartet is required for normal development, as removal of all four micromeres prevented dorsoventral axis formation. In most cases, when either one or two adjacent first quartet micromeres were removed from one side of the embryo, the cell quadrant on the opposite side, with its macromere centered under the greatest number of the remaining animal micromeres, ultimately became the D quadrant. Twins containing duplicated dorsoventral axes were generated by removal of two opposing first quartet micromeres. Thus, any cell quadrant can become the D quadrant, and the dorsoventral axis is established after the eight-cell stage. While it is not yet clear exactly when key inductive interactions take place that establish the D quadrant in C. lacteus, contacts between the progeny of animal micromeres and vegetal macromeres are established during the interval between the fifth and sixth round of cleavage divisions (i.e., 32- to 64-cell stages). These findings argue that this mechanism of cell and axis determination has been conserved among equal-cleaving spiralians.

Embryonic development in the primitive bilaterian Neochildia fusca: normal morphogenesis and isolation of POU genes Brn-1 and Brn-3.

Neochildia fusca is a member of the taxon Acoela, a group of flatworms that, according to some recent molecular phylogenetic analyses, are distinct from other flatworms and constitute a basal branch with a sister taxon relationship to the rest of the Bilateria. In this paper, we analyze early neural development in this species and report the sequence and expression of two Pit-Oct-Unc (POU) genes, NeocBrn-1 and NeocBrn-3. Homologs of these highly conserved genes play a role in neural fate determination in vertebrates, Drosophila and Caenorhabditis elegans. Acoels, including Neochildia, have a unique invariant pattern of early cleavage called duet spiral cleavage. In subsequent cell divisions descendants of the first three micromere duets form an outer layer of epidermal and neural progenitors surrounding the meso/endoderm progenitors, which are themselves descended from the macromere duet 4A, B and the micromere duet 4a, b. Organ formation begins at mid-embryonic stages with the epidermal primordium adopting a ciliated epithelial shape. Sub-epidermally, a bilaterally symmetric brain primordium can be seen at the anterior pole. Laterally and posteriorly, myoblasts form a thin layer underneath the epidermis. In late embryos and juveniles of Neochildia, the brain is formed by a 3-4 cell-diameter-thick layer of neurons forming a cortex surrounding a neuropile that is relatively free of cell bodies. A highly regular "orthogonal" array of muscle fibers penetrates the brain. We have isolated and partially sequenced homologs of the vertebrate Brn-1 and Brn-3 genes, which we call NeocBrn-1 and NeocBrn-3, respectively. These sequences contain and span portions of the POU-specific domain and a homeodomain, and are sequence similar to their respective homologs in vertebrates and Drosophila. RT-PCR reveals that NeocBrn-1 and NeocBrn-3 are expressed from mid-embryonic to adult stages. Whole-mount in situ hybridization shows expression of both genes in distinct subsets of nerve cells in juvenile and adult worms. NeocBrn-1 also appears in a subset of intra-epidermal gland cells. These observations are an initial step towards reconstructing the neural development of a key group of bilaterians, the Acoela. These flatworms, by virtue of their distinct morphology, development and phylogenetically basal placement, are likely to provide key insights into the interpretation of the evolution of metazoan neural architecture.

Micromere lineages in the glossiphoniid leech Helobdella.

In leech embryos, segmental mesoderm and ectoderm arise from teloblasts by lineages that are already relatively well characterized. Here, we present data concerning the early divisions and the definitive fate maps of the micromeres, a group of 25 small cells that arise during the modified spiral cleavage in leech (Helobdella robusta) and contribute to most of the nonsegmental tissues of the adult. Three noteworthy results of this work are as follows. (1) The c"' and dm' clones (3d and 3c in traditional nomenclature) give rise to a hitherto undescribed network of fibers that run from one end of the embryo to the other. (2) The clones of micromeres b" and b"' (2b and 3b in traditional nomenclature) die in normal development; the b" clone can be rescued to assume the normal c" fate if micromere c" or its clone are ablated in early development. (3) Two qualitative differences in micromere fates are seen between H. robusta (Sacramento) and another Helobdella sp. (Galt). First, in Helobdella sp. (Galt), the clone of micromere b" does not normally die, and contributes a subset of the cells arising exclusively from c" in H. robusta (Sacramento). Second, in Helobdella sp. (Galt), micromere c"' makes no definitive contribution, whereas micromere dm' gives rise to cells equivalent to those arising from c"' and dm' in H. robusta (Sacramento).

Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences.

Insight into the origin and early evolution of the animal phyla requires an understanding of how animal groups are related to one another. Thus, we set out to explore animal phylogeny by analyzing with maximum parsimony 138 morphological characters from 40 metazoan groups, and 304 18S rDNA sequences, both separately and together. Both types of data agree that arthropods are not closely related to annelids: the former group with nematodes and other molting animals (Ecdysozoa), and the latter group with molluscs and other taxa with spiral cleavage. Furthermore, neither brachiopods nor chaetognaths group with deuterostomes; brachiopods are allied with the molluscs and annelids (Lophotrochozoa), whereas chaetognaths are allied with the ecdysozoans. The major discordance between the two types of data concerns the rooting of the bilaterians, and the bilaterian sister-taxon. Morphology suggests that the root is between deuterostomes and protostomes, with ctenophores the bilaterian sister-group, whereas 18S rDNA suggests that the root is within the Lophotrochozoa with acoel flatworms and gnathostomulids as basal bilaterians, and with cnidarians the bilaterian sister-group. We suggest that this basal position of acoels and gnathostomulids is artifactal because for 1,000 replicate phylogenetic analyses with one random sequence as outgroup, the majority root with an acoel flatworm or gnathostomulid as the basal ingroup lineage. When these problematic taxa are eliminated from the matrix, the combined analysis suggests that the root lies between the deuterostomes and protostomes, and Ctenophora is the bilaterian sister-group. We suggest that because chaetognaths and lophophorates, taxa traditionally allied with deuterostomes, occupy basal positions within their respective protostomian clades, deuterostomy most likely represents a suite of characters plesiomorphic for bilaterians.

Gametogenesis, spawning behavior, and early development in the "iceworm" Hesiocaeca methanicola (Polychaeta: Hesionidae) from methane hydrates in the Gulf of Mexico

Gametogenesis, spawning behavior, and early development have been described in the methane-seep polychaete Hesiocaecamethanicola Desbruyeres and Toulmond, 1998, the first documentation of the reproductive biology of any cold-seep polychaete species. The worms were collected at the Green Canyon site in the Gulf of Mexico (27°44.77N; 91°13.33W) in August 1997. The gonads are situated in the neuropodia in association with an intraepithelial capillary system. Oogenesis is intraovarian, with oocytes remaining in the ovary until late vitellogenesis when they are released into the coelom for final growth and differentiation. Vitellogenesis involves both autosynthetic and heterosynthetic processes and, along with ovarian structure, suggests a relatively slow process of egg formation. Sperm differentiation is intertesticular until early spermiogenesis, when spermatid tetrads complete development in the coelom. The mature spermatozoon is an ect-aquasperm consisting of a tapering, digitate acrosome, a spherical nucleus, a midpiece containing from six to eight mitochondria and glycogen stores, and two centrioles associated with the flagellum. Both sexes undergo broadcast spawning, but the males spawn exclusively through the anus, a behavior previously unknown in polychaetes. Artificial fertilization resulted in spiral cleavage and development to the trochophore stage. Oocyte size-frequency analysis of adult females indicates a wide range of oocyte sizes and vitellogenic stages, suggesting that asynchronous gametogenesis occurs.

The embryonic development of the polyclad flatworm Imogine mcgrathi.

In this paper we describe the embryonic development of the polyclad flatworm Imogine mcgrathi. Imogine is an indirect developer that hatches as a planctonic Goette's larva after an embryonic period of approximately 7 days. Light and electron microscopic analyses of sections of staged embryos were combined with antibody stainings of wholemounted embryos to reconstruct the origin and movement of the primordia of the various organ systems, with particular emphasis on the nervous system. We introduce a system of morphologically defined stages aimed at facilitating future studies and cross-species comparisons among flatworm embryos. Imogine embryos undergo typical spiral cleavage. Micromere quartets 1-3 form an irregular double layer of mesenchymal cells that during gastrulation expands over micromere quartet 4. Micromere 4d divides into several large mesendodermal precursors whose position defines the ventral pole of the embryo. These cells, along with the animal micromeres that obtained a sub-surface position during cleavage, form a deep layer of cells that gives rise to all internal structures, including the nervous system, musculature, nephridia, and gut. Micromeres 4a-c are large yolky cells that are incorporated into the lumen of the gut, but do not themselves contribute to the gut epithelium. Shortly after gastrulation, cell differentiation sets in. Cells located at the surface adopt epithelial characteristics and form cilia that result in continuous movement of the post-gastrula stage embryo. Deep cells at the lateral margins of the embryo become organized into a protonephridial tube. A cluster of approximately 50 deep cells at the anterior pole forms the brain, in which we have identified sets of founder neurons of the brain commissure and the dorsal and ventral connectives. The early differentiating neurons, along with other cells forming stabilized microtubules (ciliated cells of the epidermis, gut and protonephridia; apical gland cells) could be analyzed in detail because of their labeling with an antibody against acetylated alpha-tubulin. Our findings indicate that, despite significant differences in the cleavage pattern and arrangement of blastomeres in the early embryo, morphogenesis and organ formation of a polyclad embryo follows a pattern that is very similar to the pattern observed by us and others in phylogenetically more evolved rhabdocoel flatworms.

Evolution of the bilaterian body plan: what have we learned from annelids?

Annelids, unlike their vertebrate or fruit fly cousins, are a bilaterian taxon often overlooked when addressing the question of body plan evolution. However, recent data suggest that annelids offer unique insights on the early evolution of spiral cleavage, anteroposterior axis formation, body axis segmentation, and head versus trunk distinction.

The Unique Developmental Program of the Acoel Flatworm, Neochildia fusca

Acoel embryos exhibit a unique form of development that some investigators argue is related to that found in polyclad turbellarians and coelomate spiralians, which display typical quartet spiral cleavage. We generated the first cell-lineage fate map for an acoel flatworm, Neochildia fusca, using modern intracellular lineage tracers to assess the degree of similarity between these distinct developmental programs. N. fusca develops via a “duet” cleavage pattern in which second cleavage occurs in a leiotropically oblique plane relative to the animal–vegetal axis. At the four-cell stage, the plane of first cleavage corresponds to the plane of bilateral symmetry. All remaining cleavages are symmetrical across the sagittal plane. No ectomesoderm is formed; the first three micromere duets generate only ectodermal derivatives. Endomesoderm, including the complex assemblage of circular, longitudinal, and oblique muscle fibers, as well as the peripheral and central parenchyma, is generated by both third duet macromeres. The cleavage pattern, fate map, and origins of mesoderm in N. fusca share little similarity to that exhibited by other spiralians, including the Platyhelminthes (e.g., polyclad turbellarians). These findings are considered in light of the possible evolutionary origins of the acoel duet cleavage program versus the more typical quartet spiral cleavage program. Finally, an understanding of the cell-lineage fate map allows us to interpret the results of earlier cell deletion studies examining the specification of cell fates within these embryos and reveals the existence of cell–cell inductive interactions in these embryos.

Conservation and innovation in spiralian development

It is clear that the spiralian developmental program represents a highly flexible platform for the generation of diverse larval and adult body plans. The widespread occurrence of this pattern of early development attests to its tremendous evolutionary success. Despite the large degree of conservation in the spiral cleavage pattern and other basic aspects of early development, changes in cell fate maps and in the mechanisms of blastomere specification have arisen. While we have learned a great deal about this mode of development, a number of important questions remain to be answered. To what extent do these conditions apply to the lesser studied spiralian phyla? What constraints have led to the preservation of the early spiral cleavage program? How has this developmental program been adapted for the construction of the various spiralian body plans (e.g. the segmental body plans of annelids or to the potential secondary loss of segmentation)? Are most changes associated with the elaboration of these different larval and adult body plans restricted to the late period of development? What molecular/genetic processes underlie this developmental program? Clearly, the spiralian phyla represent an important group of organisms for further studies on development and evolution.

The Cell Lineage of a Polyclad Turbellarian Embryo Reveals Close Similarity to Coelomate Spiralians

Recent molecular evidence suggests the turbellarian Platyhelminthes may represent the extant basal members of the Spiralia and therefore probably exhibit ancient features of the spiralian developmental program. The stereotypic quartet spiral cleavage pattern of the polyclad turbellarian embryo, among other features, indicates that this group may be closely related to the ancestral flatworm; however, polyclad embryos have been the subject of few experimental studies. Here we report the results of a cell lineage analysis of the embryo of the polycladHoploplana inquilinabased on microinjection of DiI into cleavage-stage blastomeres following formation of each of the four quartets of micromeres. The first quartet gives rise to most of the lateral and anterior ectoderm of the Müller's larva; the second quartet forms largely dorsal and ventral ectoderm as well as the circular muscles; the third quartet forms only small clones of ectoderm; and only the 4d cell of the fourth quartet contributes to larval structure, forming the longitudinal muscles, mesenchyme, and probably endoderm. Our results demonstrate a striking similarity between the cell lineages of polyclad and higher spiralian embryos, in which the four quadrants also bear the same relationships to the larval axes and give rise to comparable larval structures, including derivation of mesoderm from both ectodermal (2b) and endodermal precursors (4d).

Evolutionary Modifications of the Spiralian Developmental Program

SYNOPSIS. The Spiralia, an assemblage of phyla united by their stereotypic pattern of early embryonic cell divisions (spiral cleavage), is an interesting group in which to investigate the evolution of development. This paper examines modifications of developmental mechanisms within the Spiralia with emphasis on the basallybranching forms. Although demonstrating a notable degree of evolutionary conservation, the equal quartet cleavage pattern, which appears to be the ancestral condition, nonetheless exhibits modifications within the various spiralian groups, such as unequal cleavage, changes in cell size and rate of division, formation of two rather than four quadrants (duet spiral cleavage), and in extreme cases the loss of any trace of the spiral pattern. While the cell lineages of spiralians are remarkably conserved, one can discern evolutionary changes, for example in the cells that give rise to mesodenn. Studies of blastomere specification in many spiralian groups and analyses of axis determination indicate that embryos with equal versus unequal cleavage typically use different determinative mechanisms to establish cell fates and the dorsoventral axis. These properties are established early in species exhibiting unequal cleavage. While previous experiments suggested that equal cleavage was associated with late specification, there is now evidence of precocious specification of quadrant fates in some equal-cleaving species, such as the nemerteans and the polyclad turbellarians

The site of fertilisation determines dorsoventral polarity but not chirality in the zebra mussel embryo.

The dorsoventral polarity of unequally cleaving spiralian embryos becomes established at an early stage. The factors determining the position of the dorsoventral axis are still unknown. We present data showing that the sperm entry point (SEP) in both normal development and under experimental conditions determines the position of the first cleavage furrow in Dreissena embryos. The position of the spindles at second cleavage is directed by the site of fertilisation also, and the large, dorsal D quadrant of the 4-cell stage always forms opposite the SEP. The spiral chirality at third cleavage seems to be independent of both the fertilisation point and the arrangement of the quadrants. Dextral and sinistral third cleavages are found in a single egg batch, but sinistral cleavages prevail. We postulate that two factors coordinate the proper positioning of the dorsoventral axis. The sperm entry point as an epigenetic factor determines the dorsal side of the embryo. But since the dorsoventral axis forms oblique to the first cleavage furrow, this first decision is still ambiguous, and a second decision is required that, due to the alternative chirality of spiral cleavage, finally sets up the dorsoventral axis.

Morphological evidence for a morphogenetic field in gastropod mollusc eggs.

Eggs of the marine gastropod Crepidula fornicata examined by confocal imaging of FITC-lectin binding to the surface, and cryoscopic-SEM both reveal a surface architecture of linear structures organized around the animal-vegetal axis, which is spatially related to the anterior-posterior (a-p) axis of the subsequent embryo. A series of structures is also orientated with reference to specific micromere quartets formed during spiral cleavage. Thus, the surface architecture may provide a visible marker for a morphogenetic field which generates the a-p axis and organizes the cleavage pattern. Moreover, this architecture is co-extensive with that found on the vegetal, polar lobe-bearing region of eggs, as described by others, and which varies between gastropod taxa with varied types of body form. Confocal imaging reveals a distinct localization of F-actin to the architecture of the lobe region. However, the integrity of this F-actin is not responsible for the maintenance of the surface architecture. The significance of these findings to our understanding of the generation of diversity within the Gastropoda and general ontogenic mechanisms is discussed.

Early development and morphogenesis of the intracapsular veliger of Haminaea navicula (Gastropoda: Opisthobranchia: Bullomorpha)

Summary The development of Haminaea navicula was studied by anatomical and histological methods. This species shows typical spiral cleavage, which leads to a stereoblastula. Gastrulation is reached by epiboly. A fully developed veliger larva is reached within the egg capsule that is not only capable of swimming when artificially hatched but is also provided with the same larval organs (larval shell, velum, operculum, larval cephalic sensory organ, anal cells, anal gland, nephrocyst-protonephridial complex, epipodial and pallial gland cells) that are present in pelagic larvae of related species. Aspects of the development of H. navicula are compared with observations of the pelagic larvae of H. exigua and H. vesicula. Additional data from the literature are discussed.

Cleavage patterns and the topology of the metazoan tree of life.

Several major alliances of metazoan phyla have been identified by small subunit rRNA sequence comparisons. It is possible to arrange the phyla to produce a parsimonious distribution of cleavage types, requiring only one change from a radial ancestral condition to spiral cleavage and one other to "idiosyncratic" cleavage; this arrangement is consistent with most of the recent molecular phylogenies. The cleavage shifts are correlated with changes in many of the features that once were used to distinguish Protostomia and Deuterostomia. It is hypothesized that changes in cleavage direction are causally associated with changes in blastomere fates and thus that cleavage type correlates with such features as the identity of mesoderm founder cells, which in turn can constrain the mode of origination of the eucelom. Cleavage changes may also affect the timing of cell fate specification. In a tree that emphasizes cleavage parsimony, radial cleavage, regulative development, and enterocely are ancestral within the Bilateria, and spiral or idiosyncratic cleavages, mosaic development, and schizocely are associated with a change in cleavage direction. Deuterostomy is presumably ancestral and is correlated with radial cleavage for this reason, rather than mechanistically.

The early emergence of platyhelminths is contradicted by the agreement between 18S rRNA and Hox genes data

The phylogenetic position of thé platyhelminths within the metazoan tree is examined using two independent sets of molecular characters, the evolution of 18S ribosomal RNA sequences and the diversity of the genes belonging to the HOX cluster. Among the various hypotheses that have been considered by zoologists, a position of the platyhelminths within the protostomes, related to the phyla with typical spiral cleavage, appears to be favoured when taking into account all separate lines of evidence. It is in conflict with the traditional hypothesis of an early emergence at the base of the bilaterally symmetrical animals. This relatively late emergence is compatible with the old idea that flatworms are derived from a coelomate ancestor. New evidence from the sequences of Hox. genes suggests that the duplicated genes Ultrabithorax/abdominal-A constitute a genetic synapomorphy of the whole protostome clade.

Dual Origins of Mesoderm in a Basal Spiralian: Cell Lineage Analyses in the Polyclad TurbellarianHoploplana inquilina

Evolutionary modifications in the origins and organization of the mesoderm represent significant events in the diversification of metazoan body plans. Within the Spiralia, mesoderm comprises ectomesoderm, which typically is generated by derivatives of the second and third quartets of micromeres, and endomesoderm, which is formed from the fourth quartet micromere of the D quadrant (4d). It has been held that endomesoderm generates the majority of adult mesodermal derivatives, while larval mesoderm is formed primarily from ectomesoderm. The evolutionary history of these mesodermal sources could be clarified by examining basal members of the Spiralia such as the polyclad turbellarians, whose embryos exhibit canonical quartet spiral cleavage. Using the fluorescent lineage tracer DiI, we show that larval mesoderm is derived from only two cells, one from the ventral embryonic quadrant (2b, the “mesectoblast” cell), and the other from the dorsal quadrant (4d, the mesentoblast cell). We compare these results with mesodermal origins in other spiralian phyla and conclude that a dual origin of mesoderm is a primitive feature of spiralian development. We also argue that ectomesoderm and endomesoderm should not be considered as the exclusive precursors of larval and adult mesoderm, respectively.

Modified Spiral Cleavage: The Duet Cleavage Pattern and Early Blastomer Fates in the Acoel Turbellarian Neochildia fusca.

Modified Spiral Cleavage: The Duet Cleavage Pattern and Early Blastomer Fates in the Acoel Turbellarian Neochildia fusca.